The Women and Infants Transmission Study (WITS-II) will enroll HIV-infected pregnant women and their infants into a multicenter, epidemiologic cohort study whose objectives are to answer critical questions relevant to HIV disease during pregnancy, and maternal-infant transmission. In addition, research issues relevant to implementation of therapeutic intervention in this population will be addressed. The specific aims of WITS II are 1) to assess the effects of pregnancy on maternal HIV progression 2) to determine maternal co-factors related to vertical transmission 3) to evaluate techniques for early diagnosis of infected infants 4) to describe the natural history of HIV disease in infected infants and 5) to assess the feasibility of vaccine trials in this population. WITS II is a continuation of a currently funded study (WITS I). It will utilize core WITS I protocols along with modifications appropriate to the current state of knowledge to gather epidemiologic, medical, and laboratory data. A dedicated specimen repository will provide unique samples for evaluation of innovative early diagnostic techniques as well as for utilization of state of the art lab techniques in answering key research questions.